1. Field of the invention
The present invention concerns optical transmission and more specifically transmission of information by means of light pulses conveying the information and guided by optical fibers.
2. Description of the prior art
Optical fibers are subject to group velocity dispersion, also known as chromatic dispersion and referred to hereinafter for simplicity as "dispersion". This dispersion varies the speed of the spectral components of the pulse according to their optical frequency, which causes deformation of the pulse, to be more precise deformation of the temporal profile of the amplitude of the electromagnetic field of the pulse. Non-linearity in such fibers, also known as the Kerr effect, varies the relative phases of these components. Mutual compensation of this effect and of the dispersion can then occur in respect of the amplitude profile of the field of a pulse. For this to occur the pulse must have an amplitude profile shape, a phase distribution and an intensity that are suitable and dependent on the fiber, the pulse then constituting what is known as a fundamental soliton. A soliton of this kind would propagate indefinitely without deformation if this intensity were to be perfectly realized and maintained.
Unfortunately, optical fibers are also subject to losses that progressively decrease this intensity and at present it would not seem practicable to compensate such losses by continuous amplification that would make it possible to maintain a constant pulse intensity. This is why periodic amplification is applied by means of fiber amplifiers doped with erbium, for example, incorporated at regular intervals into a fiber optic line. The succession of fibers between two amplifiers is referred to hereinafter as a segment. This periodic amplification causes the intensity of the pulses to vary alternately between two extreme values at the respective ends of each segment. This intensity can be equal to that of the fundamental soliton only transiently, and distortion occurs. The main drawback of such distortion is that it leads to dispersion of some of the energy and widening of the pulse as it propagates through the segment. This can then penalize transmission.
Various arrangements have been proposed for at least limiting such distortion in the case of periodic amplification in this manner. In a first such arrangement, two conditions must be met. The first condition is that the distance between amplifiers remains less than the fundamental soliton period Zc defined at the end of this description. The second condition is that a mean intensity of the pulses is rendered equal over the segment to the intensity of the fundamental soliton. The pulse is then called an "average soliton". Its intensity varies between an initial value greater than that of the fundamental soliton at the input of each segment and a final value less than that of the fundamental soliton at the output of the segment. The use of an average soliton compensates for initial distortion near the input of the segment, and final distortion near the output of the segment. The propagation of an average soliton is described in the article: "Soliton Propagation in long fibers with Periodically Compensated Loss", L. F. Mollenauer, J. P. Gordon and M. N. Islam, IEEE Journal of Quantum Electronics, Vol. QE22, No 1, January 1986.
However, if the distance between amplifiers increases to the point where it becomes close to the soliton period, the distortion of the average soliton is accentuated. This is why a second arrangement has been proposed to limit distortion of the pulses in the case of periodic amplification. In this second prior art proposal the dispersion at each point along the length of each fiber is made approximately equal to an optimal dispersion matched to the intensity of the pulse at that point and, to be more precise, such that the pulse can continue to constitute a fundamental soliton of a fiber having this optimal dispersion. Given a substantially exponential decrease in this intensity along the segment, the optimal dispersion also decreases in a substantially exponential manner.
The second prior art proposal is to constitute the segments of a transmission system using variable dispersion fibers in which the dispersion at all points can be substantially equal to the optimal dispersion. On this aspect see the article in OPTICS LETTERS, Vol. 12, No. 1, January 1987, "Compensation of soliton broadening in nonlinear optical fibers with loss", Kazuhito Tajima, pages 54-56. Continuous longitudinal variation of the dispersion can be obtained by means of a corresponding variation in the diameter of the core of the fiber, this diameter variation in turn resulting from a temporal variation in the drawing parameters during the drawing operation that forms the fiber from a preform of much greater diameter. However, a fiber of this kind with a continuous variation of its dispersion has the drawback of a very much greater manufacturing cost per kilometer than ordinary fibers having longitudinally constant dispersion.
This is why it has also been proposed to constitute each segment of a line by a succession of fibers of constant dispersion disposed in decreasing dispersion order and choosing the dispersions so that they follow closely the decrease of the optimal dispersion. This solution is discussed in an article in OPTICS LETTERS, Vol. 19, No 3, Feb. 1, 1994, "Average soliton propagation in periodically amplified systems with stepwise dispersion-profiled fiber", W. Forysiak, F. M. Knox and N. J. Doran. It has the disadvantage of unwanted residual distortion of the pulses.
An aim of the present invention is to limit both the cost of a fiber optic line and the distortion of a pulse as it propagates in the line. Another aim of the present invention is to increase the information bit rate, the transmission distance spanned by a line of this kind and/or the spacing between the amplifiers of a line of this kind.